in large scales -- virialised, in small scales -- not --> inefficient SF:

MW SFR ~ 3 M_{\odot}/yr

Krumholz 2005

gas depletion time way smaller than Hubble time -- why stars are still forming: MHD pressure (Alfven) prevents collapse, GMCs are magnetically supercritical, but highly supersonic --> theory and observations tend to support the second explanation